I. Technical Field
The present invention relates to microwave sensors that detect intruders and the like using microwaves, and particularly relates to microwave sensors that more accurately detect only intended detection targets and avoid, as much as possible, occurrences of false alarms to improve reliability.
II. Description of Related Art
Conventionally, microwave sensors having microwaves that are transmitted toward a detection area, and when a human body (intruder) is present in the detection area, reflected waves (microwaves modulated by the Doppler Effect) from the human body are received to detect the human body are known as one type of security device (for example, see JP H07-37176A).
Further still, one type of microwave sensor that has been proposed involves measuring a distance to a detection target object, such as a human body present in the detection area, by using a plurality of microwaves of different frequencies. This type of sensor is configured so that, for example, two types of microwaves of different frequencies are transmitted toward the detection area and a phase difference in two IF signals is detected based on the respective reflected waves. The phase difference correlates to the distance to the detection target object, such that there is a tendency for greater phase differences to occur for greater distances to the detection target object. That is, it is possible to measure the distance to the detection target object by obtaining the phase difference. Furthermore, it is possible to determine whether or not the detection target object in the detection area is moving by recognizing change in the phase difference over time. In this way it becomes possible to identify only detection target objects that are moving in the detection area.
For example, when IF output signals based on reflected waves of two types of microwaves of different frequencies are sine waves IFout1 and IFout2 (having a phase difference corresponding to the distance to the detection target object) as shown in FIG. 2(a) and FIG. 2(b), rectangular waves W1 and W2 obtained by performing waveform shaping on these IF signal outputs are as shown in FIG. 3(a) and FIG. 3(b). Then, by detecting a phase difference Δφ of the rectangular waves W1 and W2 (calculated from a time difference Δt of a rising portion of the rectangular waves in the diagram), it is possible to measure a distance to the detection target object. Furthermore, by recognizing change over time in the phase difference of the rectangular waves W1 and W2, it is possible to recognize movement of the detection target object in the detection area (whether it is moving toward or away from the sensor).
In this regard, problems, such as the following, arise when using this type of sensor as a security sensor and recognizing change over time in the phase difference to recognize only the detection target object moving in the detection area.
That is, when this type of sensor is installed outside, it is possible that a phase difference will be produced in the rectangular waves W1 and W2 by a tree or a bush or the like swaying in the wind, thus causing a false alarm by inadvertently detecting the tree or bush as a detection target object. Similarly, when this type of sensor is installed inside, it is possible that a phase difference will be produced in the rectangular waves W1 and W2 also by a rotational operation of a ventilation fan, blinds or curtains or the like swaying due to the wind, or even by vibration or the like of the microwave sensor itself, and in these cases too a false alarm is produced by inadvertently detecting an object other than a human body as a detection target object.
Accordingly, the inventor of the present invention already has proposed techniques in which false alarms are avoided by accurately distinguishing between detection target objects such as human bodies and objects that are not targeted for detection, such as trees and fans and the like (see JP 2003-207462A).
These proposals involve measuring an amount of change per unit of time in a relative distance to an object (a movement distance of an object) that is present in a detection area based on reflected waves, and determining that the object is a detection target object only when the movement distance is a predetermined determination threshold or greater. That is, in contrast to the slight movement distance of a bush or tree swaying in the wind or a rotating fan, the movement distance is large for a detection target object such as a human body, and therefore by recognizing this difference a determination can be performed precisely as to whether or not the object is a detection target object. It should be noted that in the following description, these false alarm countermeasures are referred to as “bush/tree countermeasures” and the aforementioned determination threshold is referred to as “bush/tree countermeasure level”.
However, it is difficult to set the bush/tree countermeasure level appropriately. That is, when the bush/tree countermeasure level is set undesirably low (a state in which an object is detected even when the movement distance of the detected object is small), under a condition in which a bush or tree sways in the wind by a range of several tens of centimeters, the bush or tree will be inadvertently recognized as a detection target object and a false alarm will occur. In particular, this type of microwave sensor is often used in combination with a passive-type infrared sensor (PIR sensor) in which an intruder is detected from a difference between the human body and the ambient temperature after infrared rays are received from the human body in the detection area (a combined sensor), and in a case where the bush/tree countermeasure level is set low, there is a possibility that the microwave sensor will go into a state of continuously issuing alarms. When this happens, there is essentially no difference from a sensor device configured as a standalone PIR sensor, resulting in reductions in reliability as a combined sensor.
Conversely, when the bush/tree countermeasure level is set undesirably high (a state in which an object is not detected unless the total movement distance of the object is long (for example, approximately 100 cm)), it becomes difficult to perform detection on a human body moving laterally across the detection area (objects whose movement condition involves little change in relative distance from the microwave sensor), and in this case also the reliability of the microwave sensor cannot be maintained.
Accordingly, it is conceivable to automatically vary the bush/tree countermeasure level in a plurality of stages in response to factors such as conditions of the location where the microwave sensor is installed at that time. FIG. 9 is a graph showing an example of a bush/tree countermeasure level setting in a conventional technology microwave sensor. FIG. 10 is an explanatory diagram of adding and subtracting a status counter for the bush/tree countermeasure level setting. Here, a status counter SC is a counter that corresponds to an internal condition of the microwave sensor and its value is limited to a range of 0 to 100.
In the setting examples shown in these drawings, the bush/tree countermeasure levels are switchable between “none”, “weak”, and “strong”. Specifically, in an initial state and when the status counter SC is less than 30, the bush/tree countermeasure level is set to 0 m (none), when the status counter SC is 30 or more but less than 70, the bush/tree countermeasure level is set to 0.4 m (weak), and when the status counter SC is 70 or more, the bush/tree countermeasure level is set to 0.6 m (strong).
Then, in a case where the movement distance of a detected object has exceeded the bush/tree countermeasure level but it has been determined that that is a false alarm since nothing has been detected by a combined PIR sensor for example, a certain value is added to the status counter SC. By doing this, when false alarms continue a plurality of times, the value of the status counter SC increases and eventually the bush/tree countermeasure level is set one stage higher, and therefore it becomes more difficult for false alarms to occur. If, regardless of this, false alarms further continue, the bush/tree countermeasure level is set another stage higher to suppress the occurrences of false alarms. On the other hand, when a state continues in which no object is being detected, the status counter SC value is decreased gradually.
In other words, even when the bush/tree countermeasure level once becomes large as described above, if a state of no false alarms continues, the bush/tree countermeasure level is incrementally reduced and may eventually return to 0 m. By configuring in this manner, the bush/tree countermeasure level can be automatically changed among a plurality of stages and it is possible to achieve reductions in false alarms of the microwave sensor and improvements in reliability.